1. Field of the Invention
The present invention relates to a method and apparatus for displaying gray scales of a plasma display panel to prevent pseudo-contour from being generated when a moving picture is expressed on the plasma display panel in gray scales.
2. Description of the Related Art
Plasma display panels are display devices which arrange a plurality of discharge cells in a matrix and selectively make the arranged discharge cells emit light, thereby restoring video data input as an electrical signal. The plasma display panel can be driven by a DC driving method or an AC driving method according to whether the polarity of a voltage applied to maintain discharge is changed or not according to time. The plasma display panel can be classified into an opposite discharge type and a surface discharge type according to the method of arranging electrodes for generating discharge. Each type is also classified into a two-electrode structure, a three-electrode structure, or the like according to the number of electrodes installed.
FIG. 1A is a cross-sectional view of a discharge cell in a DC-type opposite discharge plasma display panel, and FIG. 1B is a cross-sectional view of a discharge cell in an AC-type surface discharge plasma display panel. As shown in FIGS. 1A (1B), the plasma display panel essentially includes discharge spaces 3 (13) between a front glass substrate 1 (11) and a rear glass substrate 2 (12). The DC-type opposite discharge display panel, as shown in FIG. 1A, fundamentally has two orthogonal electrodes 4 and 5 which are installed on the front and rear glass substrates 1 and 2, respectively. The two electrodes 4 and 5 are directly exposed to the discharge space 3, such that a discharge is sustained due to flow of electrons provided by a cathode. The AC-type surface plasma display panel, as shown in FIG. 1B, includes an address (metal) electrode 14 installed on the glass substrate 11, and a pair of discharge sustaining electrodes 15 installed on the glass substrate 12 to be orthogonal to the address electrode 14. The discharge sustaining electrodes 15 are covered by a dielectric layer 16, such that they are electrically isolated from the discharge space 13. In this case, a sustaining discharge occurs between the two discharge sustaining electrodes 15 installed within the dielectric layer 16, and is sustained by an effect (wall charge effect) due to a charge being accumulated on the surface of the dielectric layer. That is, even when a voltage lower than a discharge start voltage is applied, discharge occurs where a wall charge exists, since the discharge start voltage is the sum of the applied voltage and a wall voltage generated by the wall charges. The discharge also accumulates a negative-polarity wall charge, so that discharge is repeated and sustained where discharge occurs once.
FIG. 2 is an exploded perspective view of the three-electrode surface discharge plasma display panel shown in FIG. 1B, which is already in common use. This structure includes two discharge sustaining electrodes 15 formed parallel to each other within a discharge space formed by barrier ribs, and an address electrode 14 facing the discharge sustaining electrodes 15 to be orthogonal thereto. A fluorescent body 18 that emits red, green and blue lights by ultraviolet rays emitted during discharge, is coated within discharge spaces separated by the barrier ribs 17.
FIG. 3 is a connection diagram of electrodes of the AC-type surface discharge plasma display panel of FIG. 2. As shown in FIG. 2, several pairs of electrodes 15 are horizontally installed on a rear glass substrate, and two electrodes in each pair face each other in parallel. The electrodes 14 in strips are installed on a front glass substrate in a direction orthogonal to the electrodes 15. Here, electrodes connected commonly to each other, among the pairs of horizontal electrodes 15, are common electrodes (X electrodes), and the other electrodes separated from each other are scanning electrodes (Y electrodes). Also, electrodes perpendicular to these X and Y electrodes are address electrodes 14. In this structure, a discharge for generating wall charge to select a pixel occurs between an address electrode 14 and a scanning electrode, and thereafter, a discharge for displaying pictures repeatedly occurs for a certain time between the scanning electrode and the common electrode. The barrier ribs 17 form discharge spaces and also prevent crosstalk between adjacent pixels by blocking light generated during discharge. A plurality of unit structures are formed on one substrate in a matrix, and ultraviolet rays emitted from the respective unit structures selectively discharge a fluorescent material coated on spaces between adjacent barrier ribs, thereby accomplishing color. These unit structures act as pixels, and these pixels are collected and become a plasma display panel.
The plasma display panel having such a structure must be able to display gray scales in order to provide the performance of a color display device. Display of gray scales is accomplished using a gray scale expressing method of dividing one field into a plurality of sub-fields and time-division controlling them.
FIG. 4 shows a method of displaying a gray scale of an AC-type surface discharge plasma display panel. Here, the horizontal axis denotes time, and the vertical axis denotes the number of horizontal scan lines. In the gray scale display method of FIG.4 which is an 8-bit gray scale expression method, one field is divided into eight sub-fields, and each sub-field is comprised of an address period and a charge sustaining period. The addressing period forms a wall charge on a pair of display electrodes at a selected place on the entire screen of a plasma display panel due to selective discharge by a writing pulse, to thus write electrical-signalized information (that is, to form wall charges) between the address electrode and the scanning electrode which cross each other. The discharge sustaining period is a light emitting period which realizes image information on a real screen by discharging continuous discharge sustaining pulse between the display electrodes. The discharge sustaining period has a light emitting period ratio of 1:2:4:8:16:32:64:128. According to the principle in which a gray scale of a PDP is realized, the sub-fields are selectively driven, and at this time, emitted light is perceived for a predetermined time by the eyes of a user, so that the user perceives a gray scale as an averaged luminance. For example, in order to accomplish a gray scale of 3, an auxiliary field having a period of 1T and an auxiliary field having a period of 2T are driven, and the sum of the periods is made 3T, so that a gray scale 3 is perceived which is expressed as the amount of exposure light during a period of 3T. In the same way, a gray scale of 127 as a luminance of 127 is obtained by the amount of light exposed during a total of 127T periods by sequentially driving sub-fields having periods of 1T, 2T, 4T, 8T, 16T, 32T and 64T. When 8 sub-fields are used in this way, a total of 256 gray scales (28=256) can be displayed.
Meanwhile, FIGS. 5A through 5C are graphs for explaining a principle in which the human eye perceives a gray scale of a still picture. It is assumed that pixel A has a brightness of 127 and pixel B has a brightness of 128. In the pixel A, all sub-fields in the first half except for an auxiliary field having a period of 128T, among 8 sub-fields, emit light, and in the pixel B, only the auxiliary field in the second half having a period of 128T emits light, as shown in FIG. 5A. When these pixels are temporally at pause, the human eye senses light during a predetermined period at a certain position on the retina as shown in FIG. 5B, and thus can properly perceive the correct stimulated values, that is, brightnesses of 127 and 128, as shown in FIG. 5C.
FIG. 6 is a graph explaining a principle in which the human eye perceives a gray scale when a pixel moves. Referring to FIG. 6, if a pixel moves in a sequence of 1, 2, 4, 8, . . . , the human eye instinctively moves after this bright pixel. However, in contrast with the movement of this pixel, the human eye moves linearly and thus has a movement path such as a dotted line (B). As a consequence, the shape of the pixel landing on the retina is shown in FIG. 7A, and the luminance distribution according to a pixel on the retina is shown in FIG. 7B.
FIG. 8 is a graph showing the luminance finally perceived by the human eye when a pixel having a gray scale of 128 and a pixel having a gray scale of 127 adjacently move from left to right. In the first and second light emitting cells between 0F and 1F, sub-fields having periods of 1T, 2T, 4T, 8T, 16T, 32T and 64T stop emitting light, and only an auxiliary field having a period of 128T emits light, that is, only the second half emits light (which is indicated by the slashed portion), thus displaying a gray scale of 128. In the third and fourth light emitting cells between 0F and 1F, sub-fields having periods of 1T, 2T, 4T, 8T, 16T, 32T and 64T in the first half emit light, that is, only the first half emits light (which is indicated by the slashed portion), and an auxiliary field having a period of 128T in the second half stops emitting light, thus displaying a gray scale of 127. In this case, the human eye moves along a slanted line direction (direction B), so that the luminance distribution obtained on the retina is as shown in FIG. 9A. In this case, a discontinuous plane of brightness is generated as indicated by the slanted line (in direction B). Consequently, visual stimulation obtained on the retina is as shown in FIG. 9B, and a dark portion 0 is generated between gray scales of 128 and 127. The human eye perceives this situation as a dark band of 0 existing while a gray scale smoothly changes from a brightness of 128 to a brightness of 127. A contour that does not actually exist but is perceived by the human eye as a pixel moves, is called a pseudo contour. In accordance with this principle, a bright band of 255 is perceived by the human eye when a gray scale changes from a brightness of 127 to a brightness of 128.
FIG. 10A is a graph showing a representation of the pseudo contour phenomenon by a computer simulation when a band-shaped gray scale pattern changing from a brightness of 0 to the highest brightness of 255 in stages moves from left to right. FIG. 10B shows a variation in the luminance of a gray scale pattern when a picture is paused, wherein the horizontal axis indicates gray scales within stages of 0 to 255 and the vertical axis indicates the relative values of luminance. When a picture moves from left to right, the human eye perceives a gray scale pattern as shown in FIG. 10C. That is, some bright bands originally not existing are recognized by human""s eyes. FIG. 10D is a graph showing a variation in the luminance of this gray scale pattern, wherein abnormal peaks corresponding to the pseudo contour are generated along a line of luminance that linearly changes according to a gray scale stage.
FIGS. 11A through 11C are configuration diagrams of an auxiliary field constituted by conventional methods for reducing generation of the pseudo contour. In one conventional method for reducing generation of the pseudo contour, sub-fields 46 and 128 having a relatively long luminous time in an original auxiliary field sequence shown in FIG. 11A are divided into a plurality of identical gray scales 48 having short luminous times, as shown in FIG. 11B. In another conventional method, the sub-fields segmented, as shown in FIG. 11B, are rearranged, as shown in FIG. 11C. The method of FIG. 11C can reduce the distance of movement of light emitting portions when luminance changes, thus preventing the temporal non-uniformity of a light emitting pattern. However, according to these methods, a reduction in the pseudo contour is small, as shown in FIGS. 12A and 12B, and the pseudo contour phenomenon becomes serious with an increase in the speed, as shown in FIGS. 13A through 13D and FIGS. 14A through 14D. FIGS. 13A through 13D are graphs showing the pseudo contour when a field is divided into sub-fields, as shown in FIG. 11B, and the speeds V (=P/F) of pixels are 2, 3, 4 and 5. Referring to FIGS. 13A through 13D, the pseudo contour increases with an increase in speed, which means a degradation in the quality of image. FIG. 14A through 14D are graphs showing the pseudo contour when the sub-fields are divided and rearranged, as shown in FIG. 11C, and the speeds V (=P/F) of pixels are 2, 3, 4 and 5. Referring to FIGS. 14A through 14D, the pseudo contour increases with an increase in speed, which means a degradation in the quality of image.
As described above, these conventional pseudo contour reducing methods have weak effects so that the pseudo contour can be detected with the naked eye. Also, these conventional methods have a problem in that the pseudo contour phenomenon increases in proportion to the movement speed of a pixel.
An objective of the present invention is to provide a method and apparatus for displaying a gray scale of a plasma display panel, to reduce a pseudo contour having dark lines (or bright lines) by temporal nonuniformity which causes spacial nonuniformity at a portion of a moving picture where a gray scale change is subtle.
Accordingly, to achieve the above objective, the present invention provides a method of displaying a gray scale of a time-division plasma display panel, in which a picture in each field displayed on the plasma display panel is divided into a plurality of sub-fields such that each sub-field has a temporally different charge sustaining period, and a gray scale is thus displayed by the combination of the sub-fields, the method including the step of: dispersing and arranging image information representing a picture at an arbitrary position on one field, to each of the sub-fields constituting the field, wherein the image information position on each of the sub-fields sequentially moves from a first display position where the image information has been displayed on a field just before the field, to a third display position where the image information is expected to be displayed on a field just next to the field, via a second display position where the image information is displayed on the field.
In the present invention, it is preferable that the position of the image information displayed on each of the sub-fields is determined such that the image information sequentially moves from the first display position to the third display position according to the moving speed of image information set among the first, second and third display positions. Preferably, the image information position on each of the sub-fields is determined such that the image information sequentially moves from the first display position to the position before the second display position on sub-fields corresponding to the time for the first half of the corresponding field, and the image information sequentially moves from the second display position to the third display position on sub-fields corresponding to the time for the second half of the corresponding field. It is preferable that the image information position displayed on each of the sub-fields is set as a position where the image information moves according to control information determined by the functional relation set with respect to the characteristic values of sub-fields constituting the corresponding field. Preferably, the image information position on each of the sub-fields is determined to have an arrangement in which luminance temporally looks consistent or nearly consistent with respect to the display time of the corresponding field. It is preferable that the discharge sustaining period of each of the sub-fields is determined so as to have an arrangement in which luminance temporally looks consistent or nearly consistent with respect to the display time of the corresponding field.
In the present invention, when a field just before the previous field on which the first display position exists is called a zero order field, and a field just before the zero order field is called a xe2x88x921 order field, the motion of the zero order field or the motions of the zero order and xe2x88x921 order fields is detected, the position of image information displayed on each of the sub-fields is previously estimated by displaying the motion vector on a straight line or curve of the detected motion between the first and third display positions via the second display position, and the position of image information displayed on each of the sub-fields is determined by the estimation that the image information sequentially moves from the first display position to the third display position via the second display position.
To accomplish the above objective, the present invention provides an apparatus for displaying a gray scale of a time-division plasma display panel, the apparatus includes: a video signal input portion for separating only a pure video signal from a composite video signal; an analog-to-digital (A/D) converter for converting an analog video signal separated by the video signal input portion, into a digital video signal; a gamma correction means for correcting the video signal, suitable for the driving characteristics of a cathode ray tube, provided by the A/D converter to be suitable for the characteristics of a plasma display panel; a picture level detection means for detecting the total brightness of a picture from the gamma-corrected signal; a power controller for converting data of a video signal provided by the picture level detection means, the power controller having a power control (APC) function; a motion vector detection means for detecting the moving direction and speed of corresponding image information by the comparison of image display information received at the corresponding field with image information received at a field prior to the corresponding field, in each field of the video signal provided by the gamma correction means; a picture data rearrangement means for dispersing and rearranging pixel data provided by the power controller to several sub-fields according to the directional vector of a picture provided by the motion vector detection means; a sub-field converter for rearranging a rearranged picture signal provided by the picture data rearrangement means in each sub-field; a pulse timing control means for generating a reference timing signal of a driving pulse for driving the electrodes of a plasma display panel on the basis of a signal provided by the power controller; a discharge sustaining pulse generation means for generating a discharge sustaining pulse for driving discharge sustaining electrodes on the basis of the reference timing signal provided by the pulse timing control means; a scanning electrode driving means for directly driving scanning electrodes using the discharge sustaining pulse; an address electrode driving means for driving address electrodes using the reference timing signal provided by the pulse timing control means and a sub-field video signal provided by the sub-field conversion means; and a plasma display panel, wherein a picture on each field displayed on the plasma display panel is divided into a plurality of sub-fields, each of the sub-fields having a temporally different discharge sustaining period, and a gray scale is displayed by the combination of the different discharge sustaining periods.
In the present invention, the picture data rearrangement means includes: a means for moving the position of information displayed on each of the sub-fields at the detected moving speed and in the detected moving direction; a means for storing display information moved with respect to every information within one field; and a means for reconstructing image information for one field using the stored display information.